Semiconductor booster circuit having cascaded MOS transistors

ABSTRACT

The semiconductor booster circuit includes a plurality of stages, each of which has a MOS transistor and two capacitors. The MOS transistor, having a drain, a source and a gate, is formed in a well of a substrate portion. One capacitor has a terminal connected to the drain of the MOS transistor, while the other capacitor has a terminal connected to the gate of the MOS transistor. A first clock signal generating means generate a first clock signal via another terminal of one capacitor. A second clock signal generating mean s generate a second clock signal, with a larger amplitude than a power supply voltage, via another terminal of another capacitor. The plurality of stages are cascaded together, and in each of the stages the source of the MOS transistor is electrically connected to the well in which the transistor is formed, while the wells are electrically insulated from each other.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semi-conductor booster circuit and more particularly to a semiconductor booster circuit, such as a charge pump circuit, which is used in an EEPROM (Electrically Erasable and Programmable Read Only Memory) and a flash memory.

2. Description of the Related Art

In recent years, along with the promotion of a single 5V power supply or the promotion of a single 3V power supply for semiconductor integrated circuits such as EEPROMs and flash memories, the boosting has been performed in the integrated circuit. As a result, semiconductor booster circuits such as a Cockcroft Walton circuit and a charge pump circuit have been employed.

FIG. 18 shows a configuration of a conventional semiconductor booster circuit.

As shown in the figure, N-channel MOS transistors Q₂₀ to Q₂₄ are connected in cascade to configure a booster circuit having n stages. The gate terminals of the transistors Q₂₀ to Q₂₄ are connected to e respective source terminals N₂₀ to N₂₄ to which a clock signal φ_(A) or φ_(B) is input through respective capacitors C₂₀ to C₂₄.

As shown in FIG. 19, the clock signals φ_(A) and φ_(B) are in opposite phase with each other. Each of the clock signals φ_(A) and φ_(B) has a period of 1/f and an amplitude of V_(φ). The clock signals φ_(A) and φ_(B) are obtained from a clock signal CK through two NAND circuits ND₁ and ND₂ and three inverters IV₁ to IV₃, and the amplitude V_(φ) thereof is equal to a power supply voltage Vdd. Incidentally, in FIG. 18, reference symbol G designates a ground terminal.

As shown in FIG. 18, in this semiconductor booster circuit, the power supply voltage Vdd is output as an input signal from a source terminal N₂₇ of a transistor Q₂₅, and an output voltage V_(POUT) is output as an output signal from an output terminal N₂₆.

As described in an article “Analysis and Modeling of On-Chip High-voltage Generator Circuits for Use in EEPROM Circuits (IEEE JOURNAL OF SOLID-STATE CIRCUITS, vol. 24, No. 5, October 1989) for example, the output voltage V_(POUT) of a sort of the semiconductor booster circuit is expressed by the following expressions: V_(POUT)=V_(in)−V_(t)+n([V_(φ)·[C/(C+C_(s))−V_(t)]−I_(OUT)/f(C+C_(s))]  (1) Vt=V_(t0)+K₂·([Vbs+2_(2f))^(½)−(2_(φf))^(½))  (2) where V_(in) is an input of the booster circuit, V_(φ) is an amplitude voltage of the clock signal, f is a clock frequency, C is a coupling capacitance to the clock signal, Cs is a parasitic capacitance in each of stages in the booster circuit, n is the number of stages of the booster circuit, V_(POUT) is the output voltage in the final stage of the booster circuit, I_(OUT) is a load current in the output stage, Vto is a threshold voltage when a substrate bias is absent, Vbs is a substrate bias voltage (a potential difference between the source and a substrate or a well), φ_(f) is a Fermi potential, Vt is a threshold voltage of the transistor, and K₂ is a substrate bias coefficient.

From the expression (1), it is understood that when the load current I_(OUT) is zero and the relation of C/(C+Cs)=1 is established, the output voltage V_(POUT) is increased in proportion to both a value of (V_(φ)−Vt) and the number n of stages of the booster circuit. In the conventional booster circuit shown in FIG. 18, since the amplitude voltage V_(φ) of the clock signal is equal to the power supply voltage Vdd, the output voltage V_(POUT) is increased in proportion to both the value of (Vdd−Vt) and the number of stages of the booster circuit.

However, in the conventional booster circuit, there occurs a phenomenon that as the level of the output voltage V_(POUT) is increased, as shown in the expression (2), the threshold voltage Vt of each of the transistors Q₂₀ to Q₂₄ is increased due to the substrate effect.

Therefore, in the case where the stages of the booster circuit are discretely configured in order to prevent the substrate effect from occurring, the level of the output voltage V_(POUT) is increased in proportion to the number n of stages of the booster circuit. On the other hand, in the case where the transistors Q₂₀ to Q₂₄ are integrated to be formed on the same substrate, since the substrate effect occurs, as the number n of stages of the booster circuit is increased, the value of (Vdd−Vt) is decreased.

As a result, as shown in FIG. 20, along with the increasing of the number n of stages of the booster circuit, the output voltage V_(POUT) is decreased to a level lower than a value which is obtained when no substrate effect occurs, and is saturated at the point where the value of (Vdd−Vt) becomes zero. This means that no matter how the number n of stages of the booster circuit is increased, there is a limit in the resultant output voltage V_(POUT). FIG. 21 shows the relationship between the power supply voltage Vdd and a maximum output voltage when the number n of stages of the booster circuit is made infinitely large. When the number n of stages of the booster circuit is made infinitely large, in the case where no substrate effect occurs, the resultant output voltage V_(POUT) becomes theoretically infinite. On the other hand, in the case where the substrate effect actually occurs, the resultant output voltage V_(POUT) is limited to a value depending on the power supply voltage Vdd. That is, in the conventional booster circuit; there arises a problem that in the case where the level of the power supply voltage Vdd is low, the desired output voltage V_(POUT) can not be obtained even if the number n of stages of the booster circuit is set to any large value.

For example, in the conventional booster circuit shown in FIG. 18, in the case where the power supply voltage Vdd is 2.5V, and the threshold voltage Vto is 0.6V when no substrate effect occurs (the substrate bias voltage is 0V), when the number n of stages of the booster circuit is set to 20, 20V can be obtained as the output voltage V_(POUT). However, in the case where the power supply voltage Vdd is 2.0V, even if the number n of stages of the booster circuit is set to 100, only 12V can be obtained as the output voltage

On the other hand, in JP-A-61-254078, there is disclosed a Cockcroft type booster circuit in which a threshold voltage Vt of a MOS transistor in the subsequent stage having the substrate effect is made lower than that of a MOS transistor in the preceding stage, thereby improving the reduction of the output voltage due to the substrate effect.

However, in this configuration as well, the increase of the threshold voltage Vt due to the substrate effect can not be suppressed. For example, in the case where the level of the power supply voltage Vdd is approximately halved (Vdd=1 to 1.5V), even if the number n of stages of the booster circuit is set to any value, the desired output voltage V_(POUT) can not be obtained. In addition, since the threshold voltages Vt of the MOS transistors are set to a plurality of different levels, for example, it is necessary to conduct the extra process of photomask and ion implantation. As a result, the manufacturing process becomes complicated. This is a disadvantage.

FIG. 22 shows a configuration of still another conventional semiconductor booster circuit.

As shown in FIG. 22, eight N-channel MOS transistors M₁ to M₈ are connected in series with one another to configure a booster circuit having four stages. Gate terminals of the transistors M₁ to M₈ are connected to respective drain terminals (represented by nodes N₀ to N₇). To the drain terminals N₀, N₂, N₄ and N₆, a clock signal φ_(A) as shown in FIG. 17 is input through capacitors C₁, C₃, C₅ and C₇, respectively. To the drain terminals N₁, N₃, N₅ and N₇, a clock signal φ_(B) which is in opposite phase with the clock signal φ_(A) is input through capacitors C₂, C₄, C₆ and C₈, respectively. In addition, substrate terminals of the transistors M₁ to M₈ are connected to a ground terminal represented by a node N₂₁). In addition, both a drain terminal and a gate terminal of each of the N-channel MOS transistors M₂₀ and M₂₁ are connected to an associated input terminal (represented by a node N₂₀), and a substrate terminal thereof is connected to the ground terminal N₂₁.

That is, the node N₀ is respectively connected to the source terminal of the transistor M₂₀, both the drain terminal and the gate terminal of the transistor M₁, and one terminal of the capacitor C₁. The node N₁ is respectively connected to the source terminal of the transistor M₂₁, both the drain terminal and the gate terminal of the transistor M₂, the source terminal of the transistor M₁ and one terminal of the capacitor C₂. The node N₂ is respectively connected to both the drain terminal and the gate terminal of the transistor M₃, the source terminal of the transistor M₂ and one terminal of the capacitor C₃. The node N₃ is respectively connected to both the drain terminal and the gate terminal of the transistor M₄, the source terminal of the transistor M₃ and one terminal of the capacitor C₄. The node N₄ is respectively connected to both the drain terminal and the gate terminal of the transistor M₅, the source terminal of the transistor M₄ and one terminal of the capacitor C₅. The node N₅ is respectively connected to both the drain terminal and the gate terminal of the transistor M₆, the source terminal of the transistor M₅ and one terminal of the capacitor C₆. The node N₆ is respectively connected to both the drain terminal and the gate terminal of the transistor M₇, the source terminal of the transistor M₆ and one terminal of the capacitor C₇. In addition, the node N₇ is respectively connected to both the drain terminal and the gate terminal of the transistor M₈, the source terminal of the transistor M₇ and one terminal of the capacitor C₈. Further, an output terminal (represented by a node N₈) of this semiconductor booster circuit is connected to the source terminal of the MOS transistor M₈.

The above-mentioned expressions (1) and (2) are also applied to this booster circuit. Then, if the load current I_(OUT) is zero, the capacitance ratio C/(C+Cs) is 1, and the amplitude voltage V_(φ) of the clock signal is equal to the power supply voltage Vdd in the expression (1), the voltage which is boosted per stage is expressed by (Vdd−Vt).

Therefore, it is understood that the output voltage V_(POUT) is influenced by the margin between the threshold voltage Vt of each of the MOS transistors and the power supply voltage Vdd. In particular, it is understood that when the relation of Vt≧Vdd is established, the boosting operation is not performed in the corresponding stage. That is, if the threshold voltage Vt is increased, the voltage which is boosted per stage becomes either small or zero. Therefore, even if the number n of stages of the booster circuit is increased, the output voltage V_(POUT) is hardly or never increased. For example, since the source potential of the MOS transistor shown in FIG. 22 is equal to the output voltage V_(POUT), and the substrate potential is 0V, the substrate bias voltage Vbs is equal to the output voltage V_(POUT). Now, since the booster circuit shown in FIG. 22 is provided for generating the positive high voltage, the output voltage V_(POUT) takes one of positive values. Therefore, the threshold voltage of the MOS transistor M₈ becomes very high, and hence the boosting efficiency is reduced. This problem becomes especially pronounced during the low power source voltage operation in which the margin between the threshold voltage Vt and the power supply voltage Vdd is small.

In this booster circuit, as shown in FIG. 22, all the substrate terminals of the MOS transistors M₁ to M₈ are grounded. That is, the MOS transistors M₁ to M₈ are, as shown in FIG. 23, respectively constituted by sources/drains 454 to 462, which are formed in a P type semiconductor substrate 451, and gates 464 to 471, and the substrate terminal is connected to a ground terminal N₂₁ through a P⁺ type impurity diffusion layer 452 in the semiconductor substrate 451. Incidentally, reference numeral 453 designates a drain of a MOS transistor 20 and reference numeral 463 designates a gate of the MOS transistor 20.

Therefore, there arises a problem that the potential of the source terminal of the MOS transistor, which is located in the more backward stage, becomes higher, and the difference in the potential between the source terminal and the substrate portion is increased so that due to the so-called substrate bias effect, the threshold voltage Vt is increased, and hence the output voltage V_(POUT) is limited due to the increase of the threshold voltage Vt.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a semiconductor booster circuit in which a desired output voltage is capable of being obtained, even in the case where a level of a power supply voltage is low, without the necessity of the complicated manufacturing process.

A semiconductor booster circuit, according to the present invention, includes: a plurality of stages, each having a first MOS transistor and a first capacitor having one terminal connected to a drain terminal of the first MOS transistor, the stages being connected in series by connecting the first MOS transistors of the stages in cascade; and at least one of a first arrangement wherein a source terminal and a substrate of the first MOS transistor of each of the stages are electrically connected to each other and when the plurality of stages are divided into at least two groups, the substrates of the first MOS transistor included in each group are electrically insulated from the substrates of the first MOS transistors included in a different group and an arrangement wherein one terminal of a second capacitor is connected to a gate terminal of the first MOS transistor of each of the stages, and first clock signal generating means for inputting a first clock signal to the other terminal of the first capacitor, and second clock signal generating means for inputting a second clock signal having a larger amplitude than a power supply voltage to the other terminal of the second capacitor are provided.

The semiconductor booster circuit, according to a first aspect of the present invention, includes a plurality of stages, each having a first MOS transistor and a first capacitor having one terminal connected to a drain terminal of the MOS transistor, the stages being connected in series by connecting the MOS transistors of the stages in cascade wherein a source terminal and a substrate of the first MOS transistor of each of the stages are electrically connected to each other and when the plurality of stages are divided into at least two groups, the substrates of the first MOS transistors included in each group are electrically insulated from the substrates of the first MOS transistors included in a different group.

In one embodiment of the present invention, the first MOS transistor is a P-channel MOS transistor which is formed in an N type well region, and the N type well regions of the respective stages are electrically insulated from one another.

In one embodiment of the present invention, in each of the stages, a second capacitor having one terminal, which is connected to a gate terminal of the first MOS transistor, is provided, and also the gate terminal and the source terminal of the first MOS transistor are connected to each other through a second MOS transistor, and a gate terminal of the second MOS transistor is connected to the one terminal of the first capacitor.

In one embodiment of the present invention, a pair of first clock signals which are in opposite phase with each other are respectively inputted to the two other terminals of the first capacitors in the two continuous stages, and a pair of second clock signals which are different in pulse timing from each other are respectively inputted to the two other terminals of the second capacitors in the two continuous stages.

In one embodiment of the present invention, in each of the stages, the gate terminal of the first MOS transistor in the preceding stage is connected to the one terminal of the first capacitor in the subsequent stage, and a pair of clock signals which are in opposite phase with each other are respectively inputted to the two other terminals of the first capacitors in the two continuous stages.

In one embodiment of the present invention, each of the stages includes a first MOS transistor and a first capacitor having one terminal connected to a source terminal of the first MOS transistor, wherein the stages are connected in series by connecting the first MOS transistors of the respective stages in cascade, a gate terminal and the source terminal of the first MOS transistor in each stage are electrically connected to each other, and also the source terminal and the substrate thereof are electrically connected to each other and the substrate is electrically insulated from the substrate of the first MOS transistors in another stage.

Incidentally, in a preferred aspect of the present invention, the first MOS transistor is an N-channel MOS transistor which is formed in a P type well region, and the P type well regions of the respective stages are electrically insulated from one another.

In the first aspect of the present invention, the substrate of the MOS transistor forming each of the stages of the booster circuit is electrically insulated from the substrate of the MOS transistor of another stage, and in each of the stages, the substrate and the source terminal of the MOS transistor are electrically connected to each other, whereby the potential at the substrate of the MOS transistor is fixed to the source potential. Hence the increase of the threshold voltage of the MOS transistor due to the substrate effect is effectively suppressed.

A semiconductor booster circuit, according to a second aspect of the present invention, includes: a plurality of stages, each having a first MOS transistor, a first capacitor having one terminal connected to a drain terminal of the first MOS transistor, and a second capacitor having one terminal connected to a gate terminal of the first MOS transistor, the stages being connected in series by connecting the first MOS transistors in the respective stages in cascade; first clock signal generating means for inputting a first clock signal to the other terminal of the first capacitor and second clock signal generating means for inputting a second clock signal having a larger amplitude than a power supply voltage to the other terminal of the second capacitor.

In one embodiment of the present invention, the first clock signal includes a pair of clock signals which are in opposite phase with each other, and the pair of clock signals are respectively inputted to the two first capacitors in the two consecutive stages.

In one embodiment of the present invention, in each of the stages, the gate terminal and the drain terminal of the first MOS transistor are connected to each other through a second MOS transistor, and a gate terminal of the second MOS transistor is connected to the other terminal of the first capacitor in the subsequent stage.

In the second aspect of the present invention, in order to drive the MOS transistors to perform the boosting operation, other clock signals are employed which are different from the clock signals which are used to drive the stages and have a larger amplitude than the power supply voltage, whereby it is possible to secure the threshold for conducting the MOS transistor and also it is possible to prevent the reduction of the output voltage due to the substrate effect.

A semiconductor booster circuit, according to a third aspect of the present invention, includes: a plurality of stages, each having a first MOS transistor and a first capacitor having one terminal connected to a drain terminal of the first MOS transistor, the stages being connected in series by connecting the first MOS transistors of the respective stages in cascade, wherein a source terminal and a substrate of the first MOS transistor in each of the stages are electrically connected to each other, and when the plurality of stages are divided into at least two stages, the substrates of the first MOS transistors included in each group are electrically insulated from the substrates of the first MOS transistors included in another group; and wherein one terminal of a second capacitor is connected to a gate terminal of the first MOS transistor in each of the stages, and first clock signal generating means for inputting a first clock signal to the other terminal of the first capacitor in each stage, and second clock signal generating means for inputting a second clock signal having a larger amplitude than a power supply voltage to the other terminal of the second capacitor in each stage are provided.

In one embodiment of the present invention, the first MOS transistor is a P-channel MOS transistor which is formed in an N type well region, and the N type well regions in the respective stages are electrically insulated from one another.

In one embodiment of the present invention, in each of the stages, the gate terminal and the source terminal of the first MOS transistor are electrically connected to each other through a second MOS transistor, and a gate terminal of the second MOS transistor is connected to the one terminal of the first capacitor.

In one embodiment of the present invention, the first clock signal includes a pair of clock signals which are in opposite phase with each other, and the pair of clock signals are respectively inputted to the first capacitors in the two consecutive stages.

In the third aspect of the present invention, the substrate of the MOS transistor constituting each of the stages of the booster circuit is electrically insulated from the substrate of the MOS transistor in another stage, and also in each of the stages, the substrate and the source terminal of the MOS transistor are electrically connected to each other, whereby the potential at the substrate of the MOS transistor is fixed to the source potential. Hence the increase of the threshold voltage of the MOS transistor due to the substrate effect is suppressed.

In addition, the gate voltage of the MOS transistor which operates to perform the boosting operation in the stages is controlled by the clock signals other than the source voltage and the drain voltage, and the amplitude of each of the clock signals is made larger than the input power supply voltage of the booster circuit, whereby since even in the employment of the low power supply voltage, the MOS transistor can be sufficiently rendered to an on state, and also the voltage drop due to the threshold voltage of the MOS transistor is eliminated so that the boosting capability is improved.

A semiconductor booster circuit, according to a fourth aspect of the present invention, includes a plurality of stages, each of the stages having two first MOS transistors which are connected in series with each other and two capacitors, each having one terminal connected to a drain or source terminal of one of the first MOS transistors, the series circuits of the first MOS transistors of the respective stages being connected in series between an input side and an output side, wherein the plurality of stages are divided into at least two groups, and substrates of the first MOS transistors included in the stages of each group are formed integrally in a conductive substrate portion, and the potentials which are applied to the substrate portions of the groups are controlled independently of one another.

In one embodiment of the present invention, the booster circuit operates for generating a positive high voltage and the substrate portions of the first MOS transistors included in the more backward stage are controlled at a higher potential.

In one embodiment of the present invention, the first MOS transistor is a P-channel MOS transistor which is formed in an N type well region, and the N type well regions of the respective groups are electrically insulated from one another.

In one embodiment of the present invention, the booster circuit operates for generating a negative high voltage and the substrate portions of the first MOS transistors included in the more backward stage are controlled at a negative lower potential.

In one embodiment of the present invention, the first MOS transistor is an N-channel MOS transistor which is formed in a P type well region, and the P type well regions of the respective groups are electrically insulated from one another.

In one embodiment of the present invention, the substrate of the first MOS transistor of each stage is connected to a drain terminal or a source terminal of the first MOS transistor which is located at the preceding stage of the group to which the first MOS transistor belongs.

In one embodiment of the present invention, second capacitors each having one terminal connected to the gate terminal of one of the first MOS transistors are provided, and the gate terminal and the source or drain terminal of each of the first MOS transistors are connected to each other through a second MOS transistor, and the gate terminal of the second MOS transistor is connected to the one terminal of the first capacitor.

In one embodiment of the present invention, in each of the stages, the substrate of the second MOS transistor is connected to the substrate of the first MOS transistor.

In one embodiment of the present invention, a pair of first clock signals which are in opposite phase with each other are respectively inputted to the other terminals of the two adjacent first capacitors, and also a pair of second clock signals which are different in pulse timing from each other are respectively inputted to the other terminals of the two adjacent second capacitors.

In the fourth aspect of the present invention, since the substrate portions of the MOS transistors constituting the booster circuit are divided into groups and the potentials of the substrate portions in the respective groups are controlled independently of one another, the potentials at the substrate portions of the MOS transistors of each group can be fixed to a potential different from that of another group. Therefore, it is possible to suppress the increase of the threshold voltage of the MOS transistor due to the substrate bias effect, and also the level of the output voltage can be made higher than that in the conventional booster circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram showing a configuration of a semiconductor booster circuit according to a first embodiment of the present invention;

FIG. 2 is a circuit diagram showing a configuration of the two continuous stages of the semiconductor booster circuit according to the first embodiment shown in FIG. 1;

FIG. 3 is a timing chart showing the timing of clock pulses used in the semiconductor booster circuit shown in FIG. 1;

FIGS. 4A to 4D are respectively graphical representations showing waveforms of voltages at respective nodes of the semiconductor booster circuit shown in FIG. 1;

FIGS. 5A to 5F are respectively circuit diagrams useful in explaining the operation of the semiconductor booster circuit shown in FIG. 1;

FIG. 6 is a cross sectional view showing a device structure of the semiconductor booster circuit shown in FIG. 1;

FIG. 7 is a circuit diagram showing a configuration of a semiconductor booster circuit according to a second embodiment of the present invention;

FIG. 8 is a timing chart showing clock pulses used in the semiconductor booster circuit shown in FIG. 6;

FIG. 9 is a circuit diagram showing a configuration figuration of a semiconductor booster circuit according to a third embodiment of the present invention;

FIG. 10 is a cross sectional view showing a device structure of the semiconductor booster circuit shown in FIG. 9;

FIG. 11 is a circuit diagram showing a configuration of a semiconductor booster circuit according to a fifth embodiment of the present invention;

FIG. 12 is a timing chart showing clock pulses used in the semiconductor booster circuit shown in FIG. 11;

FIG. 13 is a circuit diagram showing a configuration of a semiconductor booster circuit according to a sixth embodiment of the present invention;

FIG. 14 is a cross sectional view showing a device structure of the semiconductor booster circuit shown in FIG. 13;

FIG. 15 is a circuit diagram showing a configuration of a semiconductor booster circuit according to a seventh embodiment of the present invention;

FIG. 16 is a timing chart showing clock pulses used in the semiconductor booster circuit shown in FIG. 13;

FIG. 17 is a timing chart showing clock pulses used in the semiconductor booster circuit shown in FIG. 15;

FIG. 18 is a circuit diagram showing a configuration of a conventional semiconductor booster circuit;

FIG. 19 is a timing chart showing clock pulses used in the conventional prior art semiconductor booster circuit;

FIG. 20 is a graphical representation showing the relationship between the number of stages and an output voltage of the conventional prior art semiconductor booster circuit;

FIG. 21 is a graphical representation showing the relationship between a power supply voltage and a maximum output voltage when the number of stages of the conventional prior art semiconductor booster circuit is infinite;

FIG. 22 is a circuit diagram showing a configuration of another conventional prior art semiconductor booster circuit; and

FIG. 23 is a cross sectional view showing a device structure of another conventional prior art semiconductor booster circuit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A first embodiment of the present invention will hereinafter be described in detail with reference to FIGS. 1 to 10.

FIG. 1 shows a configuration of a semiconductor booster circuit according to the first embodiment of the present invention.

As shown in FIG. 1, n elements of P-channel MOS transistors Q₁, Q₃, Q₅, Q₇, . . . , Q₉ are connected in cascade to configure a booster circuit having n stages. Substrate portions of the transistors Q₁, Q₃, Q₅, Q₇, . . . , Q₉ are electrically insulated from one another and also are connected to source terminals of the transistors Q₁, Q₃, Q₅, Q₇, . . . , Q₉, respectively. In addition, to drain terminals (represented by nodes N₁, N₃, N₅, N₇, . . . , N₉) a clock signal φ_(1A) or φ_(1B) which is shown in FIG. 3 is inputted through respective capacitors C₁, C₃, C₅, C₇, . . . , C₉.

In addition, to gate terminals (represented by nodes N₂, N₄, N₆, N₈, . . . , N₁₀) of the transistors Q₁, Q₃, Q₅, Q₇, . . . , Q₉, a clock signal φ_(2A) or φ_(2B) which is shown in FIG. 3 is inputted through respective capacitors C₂, C₄, C₆, C₈, . . . , C₁₀.

Further, P-channel MOS transistors Q₂, Q₄, Q₆, Q₈, . . . , Q₁₀ are respectively connected between the gate terminals N₂, N₄, N₆, N₈, . . . , N₁₀ and source terminals (represented by nodes N₃, N₅, N₇, N₁₁, . . . , N₁₂) of the transistors Q₁, Q₃, Q₅, Q₇, . . . , Q₉, and gate terminals of the transistors Q₂, Q₄, Q₆, Q₈, . . . , Q₁₀ are respectively connected to the drain terminals N₁, N₃, N₅, N₇, N₉ of the transistors Q₁, Q₃, Q₅, Q₇, . . . , Q₉.

In the booster circuit of the present embodiment, a power supply voltage Vdd is inputted as an input signal from a common source terminal (represented by a node N₀) of N-channel MOS transistors Q₁₂ and Q₁₃ to the source terminals N₁ and N₃ of the transistors Q₁ and Q₃, and an output voltage V_(POUT) is outputted as an output signal from an output terminal (represented by a node N₁₃) through an N-channel MOS transistor Q₁₁. As shown in the figure, the gate terminals of the transistors Q₁₂ and Q₁₃ are respectively connected to the source terminal N₀. In addition, to a source terminal (represented by a node N₁₂) of the transistor Q₁₁, the clock signal φ_(1A) which is shown in FIG. 3 is inputted through a capacitor C₁₁. Further, a gate terminal of the transistor Q₁₁ is connected to a drain terminal (represented by the node N₁₃).

As shown in FIG. 3, the clock signals φ_(1A) and φ_(1B) are in opposite phase with each other and have the same amplitude as the power supply voltage Vdd, and the clock signals φ_(2A) and φ_(2B) are pulse-like signals which have the amplitude equal to or larger than the power supply voltage Vdd and are in an off-state at periods when the clock signals φ_(1A) and φ_(1B) are in an on-state, respectively.

As for a clock signal generating unit 120 which operates to generate the clock signals φ_(1A) and φ_(1B) the same as the conventional unit may be employed. As for clock signal generating units 140 and 160 which operate to generate the clock signals φ_(2A) and φ_(2B) respectively, any units may be employed which operate to receive clock pulse signals CLK₂ and CLK₃ of the same timings as those of the generating timings of the clock signals φ_(2A) and φ_(2B), respectively, and control the amplitudes thereof.

Next, the description will hereinbelow be given with respect to the operation of the semiconductor booster circuit according to the first embodiment with reference to FIGS. 2 to 5.

FIG. 2 is a circuit diagram showing a configuration of two consecutive stages (a first stage and a second stage) of the semiconductor booster circuit shown in FIG. 1. In addition, FIGS. 4A to 4D show waveforms of the voltages at nodes N_(A) to N_(D) of the circuit of FIG. 2 for a time period ranging from (I) to (VI) shown in FIG. 3. Further, FIGS. 5A to 5F are respectively circuit diagrams useful in explaining the conduction state of transistors M₁ to M₄ of FIG. 2 for a time period ranging from (I) to (VI).

Firstly, for a period of time of (I), as shown in FIG. 3, the level of the clock signal φ_(1A) is raised from the ground potential up to the power supply voltage Vdd, and also the potential at the drain terminal N_(A) of the transistor M₁ shown in FIG. 2 is, as shown in FIG. 4A, raised by a voltage corresponding to the power supply voltage Vdd.

At the same time, the level of the clock signal φ_(1B) is dropped from the power supply voltage Vdd down to the ground potential 0V, and also the potential at the source terminal N_(B) of the transistor M₁ is, as shown in FIG. 4B, dropped by a voltage corresponding to the power supply voltage Vdd.

At this time, the electric charges which have been transferred from the preceding stage are accumulated in the capacitor C_(A2) which is connected to the source terminal N_(B) of the transistor M₁, and hence the potential at the source terminal N_(B) of the transistor M₁ is raised by a voltage corresponding to the electric charges accumulated in that capacitor C_(A2).

In addition, the potential at the gate terminal N_(A) of the transistor M₂ becomes higher than that at the source terminal N_(B), and hence the transistor M₂ is, as shown in FIG. 5A, switched from the on state to the off state.

At this time, as will be described later, since the P_(N) junction which is formed between the drain terminal N_(A) and the source terminal N_(B) of the transistor M₁ is biased in the forward direction, the substrate portion of the transistor M₁ which is connected to the source terminal N_(B) is maintained at the potential which is obtained by subtracting the forward bias voltage across the P_(N) junction from the potential at the drain terminal N_(A).

In addition, as shown in FIG. 4C, the potential at the gate terminal N_(C) of the transistor M₁ is dropped down to the same potential as that at the drain terminal N_(A), but the transistor M₁ remains, as shown in FIG. 5A, in the off state.

As the level of the clock signal φ_(1A) is raised from the ground potential 0V up to the power supply voltage Vdd, the potential at the source terminal N_(D) of the transistor M₃ is, as shown in FIG. 4D, raised by a voltage corresponding to the power supply voltage Vdd.

At this time, the electric charges which have been transferred from the preceding stage are accumulated in the capacitor C_(A3), and hence the potential at the source terminal ND of the transistor M₃ is raised by a voltage corresponding to the electric charges accumulated in the capacitor C_(A3).

In addition, at the time when the level of the clock signal φ_(1B) has been dropped from the power supply voltage Vdd down to the ground potential 0V, the potential at the gate terminal N_(B) of the transistor M₄ is dropped and hence the transistor M₄ is switched from the off state to the on state. Therefore, the potential at the gate terminal N_(E) of the transistor M₃ becomes the same potential as that at the source terminal N_(D) of the transistor M₃. At this time, as shown in FIG. 5A, the transistor M₃ remains in the off state.

Next, for a period of time (II), the level of the clock signal φ_(2A) is dropped from the power supply voltage Vdd down to the ground potential 0V, and hence the potential at the gate terminal N_(C) of the transistor M₁ is, as shown in FIG. 4C, dropped by a voltage corresponding to the power supply voltage Vdd.

As a result, as shown in FIG. 5B, the transistor M₁ is turned on and hence a current is caused to flow from the drain terminal N_(A) to the source terminal N_(B) of the transistor M₁ until the potential at the drain terminal N_(A) becomes equal to that at the source terminal N_(B).

That is, the electric charges are transferred from the capacitor C_(A1) to the capacitor C_(A2), and hence the potential at the drain terminal N_(A) of the transistor M₁ is, as shown in FIG. 4A, dropped, and also the potential at the source terminal N_(B) of the transistor M₁ is, as shown in FIG. 4B, raised.

In addition, with respect to the source terminal N_(D) of the transistor M₃ as well, in the same manner as that in the case of the drain terminal N_(A) of the transistor M₁, as shown in FIG. 4D, the potential at the source terminal N_(D) is dropped.

At this time, the clock signal φ_(2A) which is used to turn the transistor M₁ on is supplied from the outside through the capacitor C_(B1), and no voltage drop occurs between the drain terminal N_(A) and the source terminal N_(B) when turning the transistor M₁ on. Therefore, as compared with the prior art, the boosting capability is further improved. That is, in the above-mentioned expression (1), this state corresponds to the situation in which in the term within the brackets, Vt is equal to 0V. Thus, the boosting operation can be performed with an exceptionally good efficiency.

Next, for a period of time (III), the level of the clock signal φ_(2A) is raised from the ground potential 0V up to the power supply voltage Vdd, and hence the potential at the gate terminal N_(C) of the transistor M₁ is, as shown in FIG. 4C, raised by a voltage corresponding to the power supply voltage Vdd.

As a result, as shown in FIG. 5C, the transistor M₁ is turned off.

In addition, as shown in FIGS. 4A, 4B and 4D, the potentials at the drain terminal N_(A) and the source transistor N_(B) of the transistor M₁, and the potential at the source terminal N_(D) of the transistor M₃ do not change.

Next, for a period of time (IV), the level of the clock signal φ_(1A) is dropped from the power supply voltage Vdd down to the ground potential 0V, and hence the potential at the drain terminal N_(A) of the transistor M₁ is forced to drop by a voltage corresponding to the power supply voltage Vdd. However, in the first stage, since the transistor Q₁₂ shown in FIG. 1 goes to the on state, as shown in FIG. 4A, the potential at the drain terminal N_(A) of the transistor M₁ goes to the potential of (Vdd−Vt).

In addition, the level of the clock signal φ_(1B) is raised from the ground potential 0V up to the power supply voltage Vdd, and hence the potential at the source terminal N_(B) of the transistor M₁ is, as shown in FIG. 4B, raised by a voltage corresponding to the power supply voltage Vdd.

At this time, since the electric charges which have been transferred from the preceding stage are accumulated in the capacitor C_(A2), the potential at the source terminal N_(B) of the transistor M₁ is raised by a voltage corresponding to the electric charges accumulated in the capacitor C_(A2).

In addition, the potential at the gate terminal N_(A) of the transistor M₂ becomes lower than that at the source terminal N_(B) thereof, and hence the transistor M₂ is, as shown in FIG. 5D, switched from the off state to the on state.

As a result, the potential at the gate terminal N_(C) of the transistor M₁ is, as shown in FIG. 4C, raised up to the same potential as that at the source terminal N_(B) of the transistor M₁.

In addition, as the level of the clock signal φ_(1A) is dropped from the power supply voltage down to the ground potential 0V, the potential at the source terminal N_(D) of the transistor M₃ is, as shown in FIG. 4D, dropped by a voltage corresponding to the power supply voltage Vdd.

At this time, the electric charges which have been transferred from the preceding stage are accumulated in the capacitor C_(A3), and hence the potential at the source terminal N_(D) is raised by a voltage corresponding to the electric charges accumulated in the capacitor C_(A3).

As a result, the potential at the drain terminal N_(E) of the transistor M₄ becomes higher than that at the source terminal N_(D) thereof, and hence the transistor M₄ is, as shown in FIG. 5D, switched from the on state to the off state.

In addition, in the same manner as that in the case of the above-mentioned transistor M₁, since the PN junction which is formed between the drain terminal N_(B) and the source terminal N_(D) of the transistor M₃ is biased in the forward direction, the substrate portion of the transistor M₃ connected to the source terminal N_(D) is maintained at a voltage which is obtained by subtracting the forward bias voltage across the PN junction from the potential at the drain terminal N_(B).

Next, for a period of time of (V), the level of the clock signal φ_(2B) dropped from the power supply voltage Vdd down to the ground potential 0V, and hence the potential at the gate terminal N_(E) of the transistor M₃ is dropped by a voltage corresponding to the power supply voltage Vdd.

As a result, as shown in FIG. 5E, the transistor M₃ is turned on, and also a current is caused to flow from the drain terminal N_(B) to the source terminal N_(D) of the transistor M₃ until the potential at the drain terminal N_(B) becomes equal to that at the source terminal N_(D).

That is, the electric charges are transferred from the capacitor C_(A2) to the capacitor C_(A3), and hence as shown in FIG. 4B, the potential at the drain terminal N_(B) of the transistor M₃ is dropped and also as shown in FIG. 4D, the potential at the source terminal N_(D) of the transistor M₃ is raised.

In addition, since the transistor M₂ remains in the on state, and the potential at the gate terminal N_(C) of the transistor M₁ is equal to that at the drain terminal N_(B) of the transistor M₃, as shown in FIG. 4C, the potential at the gate terminal N_(C) of the transistor M₁ is dropped.

At this time, the clock signal φ_(2B) which is used to turn the transistor M₃ on is supplied from the outside through the capacitor C_(B2), and no voltage drop occurs between the drain terminal N_(B) and the source terminal N_(D) when turning the transistor M₃ on. Therefore, as compared with the prior art, the boosting capability is further improved.

Next, for a period of time of (VI), the level of the clock signal φ_(2B) is raised from the ground potential 0V up to the power supply voltage Vdd, and hence the potential at the gate terminal N_(E) of the transistor M₃ is raised by a voltage corresponding to the power supply voltage Vdd.

As a result, as shown in FIG. 5F, the transistor M₃ goes to the off state.

In addition, as shown in FIGS. 4A to 4D, the potentials at the nodes N_(A) to N_(D) do not change.

In the operation of prior art as described above, since the source terminals of the transistors M₁ to M₃ are boosted such that the source terminal of the transistor located in the subsequent stage becomes higher, the substrate effect acts inherently to raise, as shown in the above-mentioned expression (2), the threshold voltage Vt of each of the transistors M₁ and M₃. However, in the present embodiment, as shown in FIG. 2, since the substrate portions of the transistors M₁ and M₃ are connected to the source terminal, no substrate effect occurs, and hence the transfer of the electric charges from the preceding stage to the subsequent stage is effectively performed.

FIG. 6 is a schematic cross sectional view showing a device structure of the transistors M₁ and M₃ shown in FIG. 2.

As shown in FIG. 6, two N type well regions 11 are respectively formed in a P type semiconductor substrate 10 so as to be insulated from each other, and in each of the N type well regions 11, there is formed a MOS transistor which includes a polycrystalline silicon layer 16, as a gate electrode, which is formed on the substrate portion of the well 11 with a gate oxide film disposed therebetween, and P⁺ type impurity diffusion layers 12 as source/drain regions.

The P⁺ type impurity diffusion layer 12 of the source side in each of the transistors is electrically connected to the N type well region 11, in which the transistor is formed, through a N⁺ type impurity diffusion layer 14, and the source of the transistor in the preceding stage is connected to the drain of the transistor in the subsequent stage.

By adopting this structure, the potential at the N type well region 11 as the substrate portion of each of the transistors is fixed to the source potential of each of the transistors, and hence the substrate effect can be effectively prevented from occurring.

In addition, for a period of time of (I) of FIG. 5A or (IV) of FIG. 5D, the PN junction which is formed between the P⁺ type impurity diffusion layer 12 of the drain side and the N type well region 11 of each of the transistors is biased in the forward direction. Then, through that PN junction, the electric charges can be transferred from the node N_(A) to the node N_(B) and from the node N_(B) to the node N_(D) through the N type well region 11 of the substrate and the N⁺ type impurity diffusion layer 14. In this case, the voltage difference corresponding to the forward bias voltage V_(F) (normally, about 0.7V) across the PN junction which is independent of the threshold voltage Vt of the MOS transistor is utilized for the boosting operation, and hence V_(F) is employed instead of Vt in the above-mentioned expressions (1) and (2). Since the forward bias voltage V_(F) across the PN junction is not influenced by the substrate effect, it is possible to realize the booster circuit which, even when the number of stages of the booster circuit is increased, is free from the reduction of the boosting capability due to the substrate effect.

As described above, in the semiconductor booster circuit according to the first embodiment of the present invention, the substrate portions of the MOS transistors Q₁, Q₃, Q₅, Q₇, . . . , Q₉ shown in FIG. 1 are electrically insulated from one another, and also the substrate portions are respectively connected to the source terminals N₃, N₅, N₇, N₁₁, . . . , N₁₂, whereby the increase of the threshold voltage Vt due to the substrate effect is effectively prevented. Therefore, it is possible to obtain the output voltage V_(POUT) which is increased in proportion to the number n of stages of the booster circuit and also it is possible to provide the semiconductor booster circuit which has the higher boosting capability than that of the prior art.

In addition, with respect to the structure of the present embodiment, as shown in FIG. 6, the N type well regions 11 in which the respective transistors are formed are isolated from each other and also the N⁺ type impurity region 14 of each of the N type well regions 11 is electrically connected to the P⁺ type impurity region 12 of the source side of each of the transistors. Thus, the process for making the threshold voltages of the respective transistors different from one another, as in the prior art, is not required at all. Therefore, the number of manufacturing processes is not increased much.

In addition, the substrate portion of each transistor is electrically connected to the source terminal, whereby the PN junction which is formed in the boundary between the drain and the substrate portion is connected in parallel between the source and drain of the transistor. Then, when transferring the electric charges from the preceding stage to the subsequent stage in the booster circuit, the PN junction is biased to the on state, whereby the potential at the substrate portion of each transistor can be fixed to a voltage difference corresponding to the forward bias voltage V_(F) (normally, about 0.7V) across the PN junction. Thus, it is possible to suppress the influence of the substrate effect.

In addition, as shown in FIG. 5, to the gate terminals N_(C) and N_(E) of the transistors M₁ and M₃, the pair of clock signals φ_(2A) and φ_(2B) which are independent from the pair of clock signals φ_(1A) and φ_(1B) respectively inputted to the drain terminals N_(A) and N_(B), are respectively inputted, whereby each of the transistors M₁ and M₃ can be turned on in such a way that no potential difference occurs between the source and the drain thereof. Therefore, when transferring the electric charges from the preceding stage to the subsequent stage in the booster circuit, it is possible to realize the transfer of the electric charges such that the voltage drop does not occur which corresponds to the potential difference between the source and the drain. Therefore, since in the above-mentioned expression (1), the threshold voltage Vt can be regarded as zero, the boosting operation can be more efficiently performed as compared with the conventional booster circuit. Even the case where the number n of stages of the booster circuit and the power supply voltage Vdd are the same as those of the conventional booster circuit, the larger output voltage V_(POUT) can be obtained. In addition, in the case where the output voltage V_(POUT) is the same as that of the conventional booster circuit, the booster circuit of the present embodiment can obtain the larger load current I_(OUT).

For example, in the case where the power supply voltage Vdd is 20V, and the number n of stages of the booster circuit is 20, assuming that the capacitance ratio C/(C+Cs) is 0.9, the absolute value of the threshold voltage |Vt| is 0.6, and the load current I_(OUT) of the output stage is zero, only 20V can be obtained as the output voltage V_(POUT) in the conventional booster circuit, but in the booster circuit according to the present embodiment, about 47V can be obtained.

In addition, in the semiconductor booster circuit according to the present embodiment, even in the low power supply voltage Vdd which can not be boosted by the conventional booster circuit, the desired output can be obtained. In other words, in the conventional booster circuit, as shown in FIG. 21, even if the number n of stages of the booster circuit is set to any value, the maximum output voltage is limited to a predetermined value depending on the power supply voltage Vdd. However, in the semiconductor booster circuit according to the present embodiment, such a limit is not present substantially.

For example, in the case where the power supply voltage Vdd is 2.0V, assuming that the capacitance ratio C/(C+Cs) is 0.9, the absolute value of the threshold voltage |Vt| is 0.6V, and the load current I_(OUT) in the output stage is zero, even in the booster circuit in which the number n of stages of the booster circuit is 50, only 12V can be obtained as the output voltage V_(POUT) in the conventional booster circuit. In the booster circuit according to the present embodiment, when the number n of stages of the booster circuit is 20, about 37V can be obtained as the output voltage V_(POUT), and also when the number n of stages of the booster circuit is 50, about 91V can be obtained.

Incidentally, in the semiconductor booster circuit according to the present embodiment, in the case where the absolute value of the threshold voltage |Vt| is set to 0.6V, the lower limit of the boostable power supply voltage Vdd is about 0.7V.

In the above explanation, the substrates of the MOS transistors in n stages are electrically insulated from each other. Alternatively, the n stages are divided into at least two groups, for example two groups i.e. a first group of the first to third stages and a second group of the fourth to sixth stages. The substrates of the MOS transistors included in each group are electrically insulated form the substrates of the MOS transistors included in the other group.

Next, the description will hereinbelow be given with respect to a semiconductor booster circuit according to a second embodiment of the present invention with reference to FIGS. 7 and 8.

FIG. 7 is a circuit diagram showing a configuration of the semiconductor booster circuit according to the second embodiment of the present invention.

In FIG. 7, n elements of P-channel MOS transistors Q₃₀ to Q₃₄ are connected in cascade to configure a booster circuit having n stages. Substrate portions of the respective transistors Q₃₀ to Q₃₄ are electrically insulated from one another, and also gate terminals and the substrate portions are connected to respective source terminals N₃₁ to N₃₅. Then, a clock signal φ_(A) or φ_(B) which is shown in FIG. 8 is inputted to the source terminals N₃₀ to N₃₅ through capacitors C₃₀ to C₃₅, respectively.

In the booster circuit of the present embodiment, as an inputted signal, the power supply voltage Vdd is inputted from a source terminal N₃₇ of a P-channel MOS transistor Q₃₆ to a drain terminal N₃₀ of the transistor Q₃₀. As an output signal, an output voltage V_(POUT) is outputted from an output terminal N₃₆ through a P-channel MOS transistor Q₃₅.

The clock signals φ_(A) and φ_(B) are, as shown in FIG. 8, in opposite phase with each other and have the amplitude of a voltage V_(φ).

In addition, the device structure of the transistors Q₃₀ to Q₃₄ of the present embodiment may be the same as that shown in FIG. 6. That is, the N type well regions 11 are formed in the P type semiconductor substrate, and in each of the N type well regions 11, the polycrystalline silicon layer 16 which is formed on the substrate portion of the well region 11 with an intermediate gate oxide film 15 therebetween is provided as the gate electrode. and also the P⁺ type impurity diffusion layer 12 is provided as the source/drain region. In such a manner, the MOS transistor is formed.

The P⁺ type impurity diffusion layer 12 of the source side of each of the transistors is connected to the N type well region 11 through the N⁺ type impurity diffusion layer 14, and also the source of the transistor in the preceding stage is connected to the drain of the transistor in the subsequent stage. As a result, the potential at the N type well region as the substrate portion of each of the transistors is fixed to the source potential of each of the transistors, and hence the substrate effect is effectively prevented from occurring.

In addition, the PN junction, which is formed between the P⁺ type impurity diffusion layer 12 of the drain side and the N type well region of each of the transistors, is biased in the forward direction, whereby through that PN junction, the electric charges are transferred from the node N_(A) to the node N_(B) and from the node N_(B) to the node N_(D) through the N type well region 11 of the substrate portion and the N⁺ type impurity diffusion layer 14. In the case of the present embodiment, each transistor is not rendered substantially perfectly conductive unlike the state as shown in FIGS. 5B and 5E of the above-mentioned first embodiment, and hence the transfer of the electric charges from the preceding stage to the subsequent stage is performed through the above-mentioned PN junction. Therefore, in the case of the present embodiment, the potential difference corresponding to the forward bias voltage V_(F) (normally, about 0.7V) across the PN junction which is independent of the threshold voltage Vt of the MOS transistor is utilized for the boosting operation, and also V_(F) is employed instead of Vt in the above-mentioned expressions (1) and (2). Since that forward bias voltage V_(F) across the PN junction is not influenced by the substrate effect at all, it is possible to realize the booster circuit in which even when the number of stages of the booster circuit is increased, the reduction of the boosting capability due to the substrate effect does not occur at all.

More specifically, in the present embodiment, as shown in FIG. 7, the substrate portions of the transistors Q₃₀ to Q₃₄ are electrically connected to the source terminals N₃₁ to N₃₅, respectively, whereby the PN junction which is formed in the boundary between the drain and the substrate portion is connected in parallel between the source and the drain of each of the transistors Q₃₀ to Q₃₄. Then, when transferring the electric charges from the preceding stage to the subsequent stage in the booster circuit, the PN junction is rendered conductive, whereby the potential at the substrate portion of each of the transistors Q₃₀ to Q₃₄ can be fixed to the potential difference corresponding to the forward bias voltage V_(F) (normally, about 0.7V) across the PN junction. Therefore, it is possible to suppress the influence of the substrate effect.

Next, the description will hereinbelow be given with respect to a semiconductor booster circuit according to a third embodiment of the present invention with reference to FIGS. 9 and 10.

FIG. 9 is a circuit diagram showing a configuration of the semiconductor booster circuit according to the third embodiment of the present invention.

In FIG. 9, N-channel MOS transistors Q₄₀ to Q₄₄ are connected in cascade to configure the booster circuit having n stages. Substrate portions of the respective transistors Q₄₀ to Q₄₄ are electrically insulated from one another, and also the substrate portions and gate terminals are connected to respective source terminals N₄₀ to N₄₄. Then, the clock signal φ_(A) or φ_(B) which is the same as that shown in FIG. 8 is inputted to the terminals N₄₀ to N₄₄ through capacitors C₄₀ to C₄₄, respectively.

In the booster circuit according to the present embodiment, the power supply voltage Vdd is inputted as an input signal from a source terminal N₄₇ of an N-channel MOS transistor Q₄₅ to the terminal N₄₀, and also th e output voltage V_(POUT) is outputted as an output signal from an output terminal N₄₆ through the N-channel MOS transistor Q₄₄.

FIG. 10 shows a device structure of the transistors Q₄₀ to Q₄₄ according to the present embodiment.

In FIG. 10, P type well regions 51 are formed in an N type semiconductor substrate 50, and in each of the P type well regions 51, a polycrystalline silicon layer 56 which is formed on the substrate portion of the well region 11 with a gate oxide film 55 disposed therebetween is provided as a gate electrode, and also an N⁺ type impurity diffusion layer 52 is provided as a source/drain region. In such a manner, the MOS transistor is formed.

The N⁺ type impurity diffusion layer 52 of the source side of each of the transistors is electrically connected to the P type well region 51, in which the transistor is formed, through the P⁺ type impurity diffusion layer 54, and the source of the transistor in the preceding stage is connected to the drain of the transistor in the subsequent stage.

As a result, the potential at the P type well region of the substrate portion of each of the transistors is fixed to the source potential of each of the transistors, and hence the substrate effect can be effectively prevented from occurring.

In addition, the PN junction is formed between the N⁺ type impurity diffusion layer 52 of the drain side and the P type well region 51 of each of the transistors. When in the operation, the PN junction is biased in the forward direction, the potential at the substrate portion of each of the transistors is fixed to the forward bias voltage across the PN junction. In such a manner, the substrate effect can be effectively prevented from occurring.

As described above, in the semiconductor booster circuit according to the third embodiment of the present invention, the substrate portions of the MOS transistors are electrically insulated from one another, and also the substrate portions are electrically connected to the source terminals of the MOS transistors, respectively, whereby it is possible to prevent the increase of the threshold voltage Vt due to the substrate effect. Therefore, it is possible to obtain the output voltage V_(POUT) proportional to the number n of stages of the semiconductor booster circuit.

In addition, with respect to the device structure, as shown in FIG. 10, the P type well regions 51 in which the transistors Q₄₀ to Q₄₄ are respectively formed are formed independently of each other and also the P⁺ type impurity diffusion region of each of the P type well regions is electrically connected to the N⁺ type impurity diffusion region 52 of the source side of each of the transistors Q₄₀ to Q₄₄. Therefore, in particular, the number of manufacturing processes is not increased at all.

In addition, the substrate portions of the transistors Q₄₀ to Q₄₄ are electrically connected to the source terminals N₄₀ to N₄₄, respectively, whereby the PN junction which is formed in the boundary between the drain and the substrate portion is connected in parallel between the source and the drain of each of the transistors Q₄₀ to Q₄₄. Then, when transferring the electric charges from the preceding stage to the subsequent stage in the booster circuit, the PN junction is switched to the on state, whereby the potential at the substrate portion of each of the transistors Q₄₀ to Q₄₄ can be fixed to the potential difference corresponding to the forward bias voltage V_(F) (normally, about 0.7V) across the PN junction. Thus, it is possible to suppress the influence of the substrate effect.

In the semiconductor booster circuits according to the second and third embodiments of the present invention, the forward junction bias voltage V_(F) can be employed instead of the threshold voltage Vt in the above-mentioned expressions (1) and (2). In particular, in the case where the threshold voltage Vt is larger than the forward junction bias voltage V_(F), since the voltage drop when transferring the electric charges from the preceding stage to the subsequent stage in the booster circuit is reduced, it is possible to improve the boosting capability of the booster circuit. That is, the voltage drop when the electric charges are transferred to the subsequent stage depends on the smaller one of the threshold voltage Vt and the forward junction bias voltage V_(F).

For example, in the case where the power supply voltage Vdd is 2.5V and the number n of stages of the booster circuit is 20, assuming that the capacitance ratio C/(C+Cs) is 0.9, the absolute value of the threshold voltage |Vt| is 0.6V, the load current I_(OUT) in the output stage is 0 A, and the forward junction bias voltage V_(F) across the PN junction is 0.7V, only 20V can be obtained as the output voltage V_(POUT) in the conventional booster circuit. But in the booster circuit according to the third embodiment of the present invention, about 33V can be obtained as the output voltage V_(POUT).

In addition, for example, in the case where the power supply voltage Vdd is 2.0V, assuming that the capacitance ratio C/(C+Cs) is 0.9, the absolute value of the threshold voltage |Vt| is 0.6V, the load current I_(OUT) in the output stage is 0 A, and the forward junction bias voltage V_(F) across the PN junction is 0.7V, only 12V can be obtained as the output voltage V_(POUT) in the conventional booster circuit, even when the number n of stages of the booster circuit is 50. But in the booster circuit according to the third embodiment of the present invention, when the number n of stages of the booster circuit is 20, about 23V can be obtained as the output voltage V_(POUT), and also when the number n of stages of the booster circuit is 50, about 56V can be obtained.

In the semiconductor booster circuits according to the second and third embodiments of the present invention, assuming that the forward junction bias voltage V_(F) across the PN junction is 0.7V, and the capacitance ratio C/(C+Cs) is 0.9, the lower limit of the boostable power supply voltage Vdd is about 0.8V.

In the above, the description has been given with respect to the first, second and third embodiments of the present invention since in the booster circuit according to the first embodiment, the voltage drop when transferring the electric charges to the subsequent stage can be made substantially zero, and hence, the booster circuit according to the first embodiment has the larger boosting capability as compared with the booster circuits according to the second and third embodiments. In particular, in the power supply voltage Vdd of about 0.8V to 2.0V, the difference in the boosting capability between the booster circuit according to the first embodiment and the booster circuit according to the second or third embodiment becomes remarkably large.

In particular, in the power supply voltage Vdd of about 0.8V to 2.0V, when the desired output voltage is larger, the number n of stages needs to be increased in the booster circuits according to the second and third embodiments due to the voltage drop when transferring the electric charges to the subsequent stage. However, in the booster circuit according to the first embodiment, this is not required. For example, in the case where the power supply voltage Vdd is 2.0V, the number n of stages of the booster circuit required for obtaining 23V as the output voltage V_(POUT) is 20 in the booster circuits according to the second and third embodiments, but only 12 in the booster circuit according to the first embodiment.

On the other hand, the booster circuit according to the second or third embodiment is advantageous as compared with the booster circuit according to the first embodiment in that the circuit configuration is simpler and also only two kinds of clock signals are sufficient.

In any one of the above-mentioned embodiments, since the substrate portions of the MOS transistors are electrically insulated from one another, and also the substrate portions are electrically connected to the source terminals of the MOS transistors, respectively, the substrate effect can be effectively prevented from occurring . Therefore, the high boosting capability can be obtained.

In addition, no complicated manufacturing process is especially required.

Further, in the case where the same boosting capability is obtained, the number of stages of the booster circuit can be further reduced as compared with the prior art.

Therefore, in the above-mentioned expression (1), the threshold voltage Vt can be regarded as zero, and therefore, as compared with the conventional booster circuit, the boosting operation can be more efficiently performed. Thus, even in the case where the number n of stages of the booster circuit, and the power supply voltage Vdd are the same as those of the conventional booster circuit, it is possible to obtain a larger output voltage V_(POUT) than that of the conventional booster circuit.

For example, in the case where the power supply voltage Vdd is 2.5V, and the number n of stages of the booster circuit is 20, assuming that the capacitance ratio C/(C+Cs) is 0.9, the absolute value of the threshold voltage |Vt| is 0.6V, the load current I_(OUT) in the output stage is zero, and the boosted voltage Vhh is 3.0V, only 20V can be obtained as the output voltage V_(POUT) in the conventional circuit, but in the booster circuit according to the present embodiment, about 47V can be obtained as the output voltage V_(POUT)

This means that in the case where the output voltage V_(POUT) is the same, the booster circuit according to the present embodiment can provide a larger load current I_(OUT) than that in the conventional circuit.

In addition, in the booster circuit according to the present embodiment, as can be seen from FIG. 10, even with a low power supply voltage Vdd which can not be boosted by the conventional circuit, the desired output voltage can be obtained.

For example, assuming that the capacitance ratio C/(C+Cs) is 0.9, the absolute value of the threshold voltage |Vt| is 0.6V, the load current I_(OUT) in the output stage is zero, and the boosted voltage Vhh is 3.0V, the power supply voltage Vdd needs to be set to 2.5V or more, in the conventional booster circuit, in order to obtain 20V as the output voltage V_(POUT), but only 1.5V is sufficient for the power supply voltage Vdd in the booster circuit according to the present embodiment.

According to the fourth embodiment, since the clock signal which is used to render the MOS transistor constituting each of the stages conductive is boosted so as to have a larger amplitude than the power supply voltage Vdd, the desired output voltage can be obtained even in the case where the power supply voltage is low.

In addition, in the case where the power supply voltage is constant, a larger load current than that in the prior art can be obtained.

Furthermore, in the case where the same output voltage as that in the prior art is to be obtained, the number of stages of the booster circuit can be further reduced as compared with the prior art.

Next, the description will hereinbelow be given with respect to a fifth embodiment of the present invention with reference to FIGS. 11 and 12.

A circuit configuration shown in FIG. 11 is identical to that of FIG. 1 except for the provision of bootstrap circuits BS₇₁ and BS₇₂ illustrated in the lower half of the figure. Therefore, the operation of the booster circuit according to the fifth embodiment of the present invention is substantially the same as that of the booster circuit according to the first embodiment. That is, the configuration of the first and second stages which are consecutive in the booster circuit of the fifth embodiment is the same as that of FIG. 2. Then, when the clock signals φ_(1A), φ_(1B), φ_(2A) and φ_(2B) are inputted according to the timing diagram as shown in FIG. 12 in the booster circuit of FIG. 11, the state in change of the operation of each of the transistors of the booster circuit as shown in FIG. 2 and the tendency in change of the potentials at the nodes N_(A), N_(B), N_(C) and N_(D) are the same as those in the first embodiment shown in FIGS. 5A to 5F and FIGS. 4A and 4D. The only differences between the first embodiment and the fifth embodiment are as follows.

(a) For a period of time of (II) of FIG. 12, the potential at the gate terminal N_(C) of the transistor M₁ is dropped as shown in FIG. 4C. However, the voltage drop is equal to the power supply voltage Vdd in the first embodiment, but is equal to the boosted voltage Vhh in the fifth embodiment.

(b) For a period of time of (III) of FIG. 12, the potential at the gate terminal N_(C) of the transistor M₁ is raised as shown in FIG. 4C. In this connection, the raised voltage is equal to the power supply voltage Vdd in the first embodiment, but is equal to the boosted voltage Vhh in the fifth embodiment.

(c) For a period of time of (V) of FIG. 12, the potential at the gate terminal N_(E) of the transistor M₃ is dropped. In this connection, the voltage drop is equal to the power supply voltage Vdd in the first embodiment, but is equal to the boosted voltage Vhh in the fifth embodiment.

(d) For a period of time of (VI) of FIG. 12, the potential at the gate terminal N_(E) of the transistor M₃ is raised. In this connection, the raised voltage is equal to the power supply voltage Vdd in the first embodiment, but is equal to the boosted voltage Vhh in the fifth embodiment.

Next, the description will hereinbelow be given with respect to the operation of the bootstrap circuit BS₇₁ with reference to FIGS. 11 and 12.

Firstly, the level of the clock signal CLK₂ shown in FIG. 13 is changed from 0V up to Vdd. At first, the potential of φ_(2A) is changed from 0V to Vdd−Vt (Vt is the threshold voltage of the transistor Q₈₄). The threshold voltage Vt of the transistor Q₈₄ is, for example, 0.1V. When Vdd=1V, and the clock signal CLK₂ is changed from 0V to 1V, the initial potential of φ_(2A) becomes 0.7V (Vt of the transistor Q₈₄ is raised by about 0.2V due to the back bias effect). At the same time, the inverter IV₈₅ performs the inversion operation at the time when the level of the input voltage φ_(2A) has exceeded the logical threshold voltage (normally, about Vdd/2), and also the potential at the node N₉₀ is dropped from Vdd to 0V. As a result, the transistor Q₈₅ is turned on.

Next, on the basis of the function of both the inverter IV₈₄ and the capacitor C₈₃, the potential at the node N₈₇ is changed from Vdd down to 0V after a predetermined time delay from the clock signal CLK₂ and the node N₉₀. Therefore, although the transistor Q₈₆ is initially in the on-state, after a predetermined time delay, the transistor Q₈₆ is turned off. Until a lapse of the predetermined time delay, both the transistors Q₈₅ and Q₈₆ are in the on state. In this connection, by setting the on-resistance of the transistor Q₈₆ to a value sufficiently smaller than the on-resistance of the transistor Q₈₅, the potential at the node N₈₈ is maintained at about 0V until a lapse of the predetermined time delay. That is, after a lapse of the predetermined delay time, the potential at the node N₈₈ is changed from about 0V to Vdd.

Next, at the same time the potential at the node N₈₈ is changed from about 0V to Vdd, the potential of the clock signal φ_(2A) goes to (2Vdd−Vt) on the basis of the function of the capacitor C₈₂. Thus, it is possible to obtain the larger voltage than Vdd. For example, in the case of Vdd=1V, the level of the clock signal φ_(2A) settles to 1.7V.

The above description also applies to the other bootstrap circuit BS₇₂.

Therefore, by inputting the clock signals CLK₂ and CLK₃ to the bootstrap circuits BS₇₁ and BS₇₃, respectively, it is possible to obtain the clock signals φ_(2A) and φ_(2B) each having a larger amplitude than the power supply voltage Vdd.

The fifth embodiment offers basically the same effects as those of the first embodiment in that the high output voltage can be obtained. In addition, in the fifth embodiment, the level of each of the clock signals φ_(2A) and φ_(2B) is boosted by the bootstrap circuit BS₇₁ or BS₇₂ to a larger amplitude than the power supply voltage Vdd, whereby the gate voltage of each of the MOS transistors Q₁, Q₃, Q₅, Q₇, . . . , Q₉, which are connected in cascade, can be made higher than that in the prior art. Therefore, even if the threshold voltage Vt is increased due to the substrate effect, the MOS transistors Q₁, Q₃, Q₅, Q₇, . . . , Q₉ can be normally turned on, and hence it is possible to obtain the output voltage V_(POUT) which is increased in proportion to the number n of stages of the semiconductor booster circuit.

The relationship between the number of stages of the booster circuit and the output voltage is substantially the same as that in the fourth embodiment shown in FIG. 9.

In addition, in the semiconductor booster circuit according to the present embodiment, the MOS transistors Q₁, Q₃, Q₅, Q₇, . . . , Q₉ are driven by the clock signals φ_(2A) and φ_(2B) which are respectively obtained by boosting the clock signals CLK₂ and CLK₃ to a larger amplitude than the power supply voltage Vdd, whereby the MOS transistors Q₁, Q₃, Q₅, Q₇, . . . , Q₉ can be sufficiently turned on even with the very low power supply voltage value (e.g., Vdd=0.7 to 1.0V).

In the present embodiment, the lowest power supply voltage which can be boosted is determined by the threshold voltage Vt of each of the P-channel MOS transistors Q₁, Q₃, Q₅, Q₇, . . . , Q₉ constituting the booster circuit. As in the first embodiment, in the case where the amplitude V_(φ2) of each of the clock signals φ_(2A) and φ_(2B) is equal to the power supply voltage Vdd, the voltage drop at the node N_(C) in a period of time of (II) 'shown in FIG. 4C does not reach the threshold voltage Vt (e.g., −0.6V) of each of the P-channel MOS transistors Q₁, Q₃, Q₅, Q₇, . . . , Q₉ if Vdd becomes equal to or lower than 1V and hence the P-channel MOS transistors Q₁, Q₃, Q₅, Q₇, . . . , Q₉ can not be sufficiently turned on. On the other hand, as in the fifth embodiment, the amplitude V_(φ2) of each of the clock signals φ_(2A) and φ_(2B) is boosted by the bootstrap circuit BS₁ or BS₂ to a larger amplitude than the power supply voltage Vdd, whereby the boosting operation can be stably performed even in the very low power supply voltage value of Vdd=0.7 to 1.0V. In addition, since the P-channel MOS transistors Q₁, Q₃, Q₅, Q₇, . . . , Q₉ can be sufficiently turned on, it is also possible to prevent the reduction of the boosting capability of the booster circuit.

Now, the description will hereinbelow be given with respect to the relationship between the power supply voltage Vdd and the maximum output voltage V_(POUT) when making the number of stages of the booster circuit infinite with reference to FIG. 10. In the case of the conventional booster circuit, even if Vdd becomes larger than Vto, the relationship has a characteristic as shown in the curve (c) of FIG. 10 since Vt is increased due to the substrate effect with the increase of the output voltage V_(POUT). On the other hand, in the booster circuit of the present embodiment, in the case where the relation of V_(φ1)=V_(φ2)=V_(φ)=Vdd is established, and also the parasitic capacity can be disregarded i.e., for example, the relation of C/(C+Cs)=1 is established, the boosting operation can be performed depending on only the number n of stages of the booster circuit if the desired voltage equal to or higher than Vdd=V_(φ)=Vto is obtained. However, actually, since the parasitic capacity can not be disregarded, e.g. C/(C+Cs) is about 0.9, the boosting operation can not be performed when V=Vdd, unless Vdd is equal to or larger than 1.1, like the fourth embodiment. In addition in the MOS transistor, even if the gate to source voltage exceeds Vto slightly, the boosting capability of the booster circuit is slightly reduced since the source to drain resistance is large and hence the booster circuit provides a characteristic as shown in the curve (b) of FIG. 10.

On the other hand, in the structure of the fifth embodiment, since V_(φ2) can be set to 1.7 Vdd for example, it is possible to supply a gate to source voltage by which the MOS transistor can be sufficiently turned on even if the parasitic capacity is present. As a result, the fifth embodiment provides a characteristic as shown in the curve (a) of FIG. 10.

Next, the description will hereinbelow be given with respect to a sixth embodiment of the present invention with reference to FIGS. 13 and 14.

FIG. 13 shows a configuration of a semiconductor booster circuit according to the sixth embodiment of the present invention.

As shown in FIG. 13, N-channel depletion type MOS transistors M₁₀₁ to M₁₀₈ are connected in series between an input terminal N₁₂₀ and an output terminal to configure a booster circuit having four stages. That is, each pair of the transistors M₁₀₁ and M₁₀₂; M₁₀₃ and M₁₀₄; M₁₀₅ and M₁₀₆; M₁₀₇ and M₁₀₈ constitute respective stages. Gate terminals of the transistors M₁₀₁ to M₁₀₈ are respectively connected to drain terminals (represented by nodes N₁₀₀ to N₁₀₇). Then, a clock signal φ_(A) which is shown in FIG. 16 is inputted through capacitors C₁₀₁, C₁₀₃, C₁₀₅ and C₁₀₇, respectively, to the drain terminals N₁₀₀, N₁₀₂, N₁₀₄ and N₁₀₆ and also a clock signal φ_(B) which is in opposite phase with the clock signal φ_(A) is inputted through capacitors C₁₀₂, C₁₀₄, C₁₀₆ and C₁₀₈, respectively, to the drain terminals N₁₀₁, N₁₀₃, N₁₀₅ and N₁₀₇. In addition, both drain terminals and gate terminals of N-channel MOS transistors M₁₂₀ and M₁₂₁ are connected to the input terminal (represented by a node N₁₂₀), and substrate terminals thereof are connected to a ground terminal (represented by a node N₁₂₁).

In addition, substrate terminals of the transistors M₁₀₁ to M₁₀₈ are divided into two groups, as will be described later, i.e., the group of the transistors M₁₀₁ to M₁₀₄ and the group of the transistors M₁₀₅ to M₁₀₈. In this connection, the substrate terminals of the transistors M₁₀₁ to M₁₀₄ and the substrate terminals of the transistors M₁₀₅ to M₁₀₈ are respectively connected to the drain terminal N₁₀₄ of the transistor M₁₀₁ and the drain terminal N₁₀₄ of the transistor M₁₀₅.

That is, the node N₁₀₀ is connected to the source terminal of the transistor M₁₂₀, both the drain terminal and the gate terminal of the transistor M₁₀₁, one terminal of the capacitor C₁₀₁ and the substrate terminals of the transistors M₁₀₁ to M₁₀₄. The node N₁₀₁ is connected to the source terminal of the transistor M₁₂₁, both the drain terminal and the gate terminal of the transistor M₁₀₂, the source terminal of the transistor M₁₀₁ and one terminal of the capacitor C₁₀₂. The node N₁₀₂ is connected to both the drain terminal and the gate terminal of the transistor M₁₀₃, the source terminal of the transistor M₁₀₂ and one terminal of the capatcitor C₁₀₃. The node N₁₀₃ is connected to both the drain terminal and the gate terminal of the transistor M₁₀₄, the source terminal of the transistor M₁₀₃ and one terminal of the capacitor C₁₀₄. The node N₁₀₄ is connected to both the drain terminal and the gate terminal of the transistor M₁₀₅, the source terminal of the transistor M₁₀₄, one terminal of the capacitor C₁₀₅ and the substrate terminals of the transistors M₁₀₅ to M₁₀₈. The node N₁₀₅ is connected to both the drain terminal and the gate terminal of the transistor M₁₀₆, the source terminal of the transistor M₁₀₅ and one terminal of the capacitor C₁₀₆. The node N₁₀₆ is connected to both the drain terminal and the gate terminal of the transistor M₁₀₇, the source terminal of the transistor M₁₀₆ and one terminal of the capacitor C₁₀₇. In addition, the node N₁₀₇ is connected to both the drain terminal and the gate terminal of the transistor M₁₀₈, the source terminal of the transistor M₁₀₇ and one terminal of the capacitor C₁₀₈. Further, he output terminal of the semiconductor booster circuit is connected to the source terminal of the transistor M₁₀₈.

In this configuration, the series-connected our stages are divided into a group of two stages of he input side including the transistors M₁₀₁ to M₁₀₄ and group of two stages of the output side including the transistors M₁₀₅ to M₁₀₈. Therefore, the substrate terminals of the transistors M₁₀₁ to M₁₀₈ are divided into the group of substrate terminals of the transistors M₁₀₁ to M₁₀₄ and the group of substrate terminals of the transistors M₁₀₅ to M₁₀₈. In this connection, the substrate terminals of the transistors M₁₀₁ to M₁₀₄ are connected to the drain terminal N₁₀₀ of the transistor M₁₀₁ and the substrate terminals of the transistors M₁₀₅ to M₁₀₈ are connected to the drain terminal N₁₀₄ of the transistor M₁₀₅. Therefore, as compared with the conventional booster circuit shown in FIG. 22, a substrate bias voltage Vbs of the transistors M₁₀₁ to M₁₀₇ in the booster circuit of the present embodiment is smaller than that of the transistors M₁ to M₇ in the conventional booster circuit. As a result, the threshold voltage Vt of the transistors M₁₀₅ to M₁₀₈ in the booster circuit of the present embodiment is lower than that of the transistors M₅ to M₈ in the conventional booster circuit. As a result, as compared with the conventional booster circuit, the boosting capability is further improved in the booster circuit of the present embodiment so that the high output voltage is obtained, and also the number of stages required for obtaining the same output voltage can be further reduced, as compared with the conventional booster circuit. In addition, since the threshold voltage Vt in each of the stages is lowered, the lower limit of the boostable power supply voltage Vdd becomes small, and hence the drive with the low power supply voltage becomes possible.

Next, the description will hereinbelow be given with respect to the device structure of the booster circuit shown in FIG. 13 with reference to FIG. 14 .

As shown in FIG. 14, in an N type well region 402 which is formed in a P type semiconductor substrate 401, P type well regions 403, 404 and 405 are respectively formed. P⁺ type impurity diffusion layer 406 and N⁺ type impurity diffusion layers 409 and 410 are respectively formed in the P⁺ type well region 403, and also a polycrystalline silicon film 421 as a gate electrode is formed above a channel region between the N type impurity diffusion layers 409 and 410 as the drain and the source with an interposed gate oxide film (not shown), thereby constituting the transistor M₁₂₀. In addition, a P⁺ type impurity diffusion layer 407 and N⁺ type impurity diffusion layers 411 to 415 are respectively formed in the P type well region 404, and also polycrystalline silicon films 422 to 425 as gate electrodes of the transistors are formed above channel regions between N⁺ type impurity diffusion layers 411 to 415 constituting the drains or the sources of the transistors with an intermediate gate oxide film (not shown), thereby constituting the four transistors M₁₀₁ to M₁₀₄. In addition, a P⁺ type impurity diffusion layer 408 and N⁺ type impurity diffusion layers 416 to 420 are respectively formed in the P type well region, and also polycrystalline silicon films 426 to 429 as gate electrodes of the transistors are formed above channel regions between N⁺ type impurity diffusion layers 416 to 420 constituting the drains or the sources of the transistors with an intermediate gate oxide film (not shown), thereby constituting the four transistors M₁₀₅ to M₁₀₈.

The polycrystalline silicon films 422 to 425 as the gate electrodes of the transistors M₁₀₁ to M₁₀₄ are respectively connected to the N⁺ type impurity diffusion layers 411 to 414, and the polycrystalline silicon films 426 to 429 as the gate electrodes of the transistors M₁₀₅ to M₁₀₈ are respectively connected to the N⁺ type impurity diffusion layers 416 to 419. In addition, the clock signal φ_(A) as shown in FIG. 4 is inputted through the capacitors C₁₀₁, C₁₀₃, C₁₀₅ and C₁₀₇, respectively, to the polycrystalline silicon films 422, 424, 426 and 428 as the gate electrodes of the transistors M₁₀₁, M₁₀₃, M₁₀₅ and M₁₀₇ and the clock signal φ_(B) which is in opposite phase with the clock signal φ_(A) is inputted through the capacitors C₁₀₂, C₁₀₄, C₁₀₆ and C₁₀₈, respectively to the polycrystalline silicon films 423, 425, 427 and 429 as the gate electrodes of the transistors M₁₀₂, M₁₀₄, M₁₀₆ and M₁₀₈. In addition, both the N⁺ type impurity diffusion layer 409 as the drain and the polycrystalline silicon film 421 as the gate electrode of the transistor M₁₂₀ are connected to the power supply terminal N₁₂₀. The P type well region 403 is connected to the ground terminal. N₁₂₁ through the P⁺ type impurity diffusion layer 406, and hence the substrate potential of the transistor M₁₂₀ is equal to the potential at the P type well region 403. In addition, the P type well region 404 is connected to both the N⁺ type impurity diffusion layer 410 as the source of the transistor M₁₂₀ and the N⁺ type impurity diffusion layer 411 as the drain of the transistor M₁₀₁ through the P⁺ type impurity diffusion layer 407, and hence the substrate potential of each of the transistors M₁₀₁ to M₁₀₄ is equal to the potential at the P type well region 404. Further, the P type well region 405 is connected to both the N⁺ type impurity diffusion layer 415 as the source of the transistor M₁₀₄ and the N⁺ type impurity diffusion layer 416 as the drain of the transistor M₁₀₅ through the P⁺ type impurity diffusion layer 408, and hence the substrate potential of each of the transistors M₁₀₅ to M₁₀₈ is equal to the potential at the P type well region 405.

Although in the embodiment described above, the substrate portions of the eight transistors M₁₀₁ to M₁₀₈ constituting the booster circuit are divided into two groups, the number of groups is not limited thereto. For example, the substrate portions are divided by every stage, and thus the four groups may be formed. But, if each division is too small, although the boosting efficiency is improved, there arises a problem that the integration of the elements can not be increased. Incidentally, although the above-mentioned embodiment has the circuit configuration having the four stages, it is to be understood that the number of stages is not limited thereto.

Next, the description will hereinbelow be given with respect to a seventh embodiment of the present invention with reference to FIGS. 15 and 17.

As shown in FIG. 15, four circuit blocks PCH₀₁ to PCH₀₄ are connected in cascade to configure the semiconductor booster circuit according to the seventh embodiment of the present invention. Each of the circuit blocks PCH₀₁ to PCH₀₄ is configured by connecting P-channel MOS transistors P₂₀₁ and P₂₀₂ in series with each other. Now, to a drain terminal N₂₀₁, of the transistor P₂₀₁, a clock signal φ_(1A) which is shown in FIG. 17 is inputted through a capacitor C₂₀₁. To a gate terminal N₂₀₃ of the transistor P₂₀₁, a clock signal φ_(2A) shown in FIG. 17 is inputted through a capacitor C₂₀₂. To a drain terminal N₂₀₂ of the transistor P₂₀₂, a clock signal φ_(1B) is inputted through a capacitor C₂₀₃. In addition, to a gate terminal N₂₀₅ of the transistor P₂₀₂, a clock signal φ_(2B) is inputted through a capacitor C₂₀₄. Further, a P-channel MOS transistor P₂₀₃ is connected between the source terminal N₂₀₄ and the gate terminal N₂₀₃ of the transistor P₂₀₁, and the gate terminal of the transistor P₂₀₃ is connected to the drain terminal N₂₀₁ of the transistor P₂₀₁. In addition, a P-channel MOS transistor P₂₀₄ is connected between the source terminal N₂₀₄ and the gate terminal N₂₀₅ of the transistor P₂₀₂, and the gate terminal of the transistor P₂₀₄ is connected to the drain terminal N₂₀₂ of the transistor P₂₀₂.

In addition, drain terminals and gate terminals of N-channel depletion type MOS transistors M₂₂₀ and M₂₂₁ are respectively connected to a power supply terminal N₂₂₀, substrate terminals thereof are connected to a ground terminal N₂₂₁, and source terminals thereof are respectively connected to the drain terminals N₂₀₁ and N₂₀₂ of the transistors P₂₀₁ and P₂₀₂ in the circuit block PCH₀₁. Incidentally, instead of the N-channel depletion type MOS transistors M₂₂₀ and M₂₂₁, N-channel enhancement type MOS transistors may also be used.

Substrate terminals of the four transistors P₂₀₁ to P₂₀₄ in the circuit blocks PCH₀₁ and PCH₀₂ are connected to a substrate terminal SUB₁ formed of a common N type well region, and th e substrate terminal SUB₁ is connected to a source terminal (not shown) of the transistor P₂₀₄ in the circuit block PCH₀₂. On the other hand, substrate terminals of four transistors P₂₀₁ to P₂₀₄ in the circuit blocks PCH₀₃ and PCH₀₄ are connected to a substrate terminals SUB₂ formed of a common N type well region and the substrate terminal SUB₂ is connected to the source terminal (not shown) of the transistor P₂₀₄ in the circuit block PCH₀₄. Incidentally, the substrate terminals SUB₁ and SUB₂ are electrically insulated from each other.

The source terminal N₂₀₄ of the transistor P₂₀₂ in the circuit block PCH₀₁ is connected to the drain terminal N₂₀₁ of the transistor P₂₀₁ in the circuit block PCH₀₂, the source terminal N₂₀₄ of the transistor P₂₀₂ in the circuit block PCH₀₂ is connected to the drain terminal N₂₀₁ of the transistor P₂₀₁ in the circuit block PCH₀₃, and the source terminal N₂₀₄ of the transistor P₂₀₂ in the circuit block PCH₀₃ is connected to the drain terminal N₂₀₁ of the transistor P₂₀₁ in the circuit block PCH₀₄ so that the four circuit blocks PCH₀₁ to PCH₀₄ are connected in cascade. In addition, the source terminal of the transistor P₂₀₂ in the circuit block PCH₀₄ is connected to an output terminal to provide the output voltage V_(POUT).

Next, the description will hereinbelow be given with respect to the operation of the semiconductor booster circuit according to the seventh embodiment of the present invention. Incidentally, in the following description, it is meant by “smaller than the threshold voltage” that the potential at the drain or the source is lower than that at the gate, or the potential at the source or the drain is higher than that at the gate, but the difference therebetween is smaller than the threshold voltage. By “larger than the threshold voltage”, it is meant that the potential at the source or the drain is higher than that at the gate and additionally the difference therebetween is larger than the threshold voltage.

Firstly, for a period of time of (I) of FIG. 17, the level of the clock signal φ_(1A) is the low potential (“L”), and the level of each of the clock signals φ_(2A), φ_(1B) and φ_(2B) are the high potential (“H”). Thus, a current is caused to flow from the power supply terminal N₂₂₀ shown in FIG. 15 to the drain terminal N₂₀₁ of the transistor P₂₀₁ through the transistor M₂₂₀, and hence the electric charges are accumulated in the capacitor C₂₀₁. The potential at the drain terminal N₂₀₂ of the transistor P₂₀₂ is higher than its previous potential existing when the level of the clock signal 1B was previously “L” by V_(φ) C/(C+Cs) (V_(φ) is the amplitude of each of the clock signals φ_(1A) and φ_(1B)) shown in the above-mentioned expression (1). Thus, if the relation in magnitude between the potential at the drain terminal N₂₀₁ of the transistor P₂₀₁ and the potential at the drain terminal N₂₀₂ of the transistor P₂₀₂ becomes larger than the threshold value of the transistor P₂₀₃, the transistor P₂₀₃ is turned on, and hence the conduction is established between the gate terminal N₂₀₃ of the transistor P₂₀₁ and the drain terminal N₂₀₂ of the transistor P₂₀₂. At this time, since the potential between the gate terminal N₂₀₃ and the drain terminal N₂₀₁ or the source terminal N₂₀₂ of the transistor P₂₀₁ is lower than the threshold voltage of the transistor P₂₀₁, the transistor P₂₀₁ is turned off. In addition, both the transistors P₂₀₂ and P₂₀₄ are turned off since the potential between the gate terminal and the drain terminal or the source terminal is lower than the threshold voltage.

Next, when the operation proceeds from a period of time of (I) to a period of time of (II), the level of each of the clock signals φ_(2A) and φ_(2B) remains “H”, and also the level of the clock signal φ_(1A) is changed from “L” to “E” and the level of the clock signal φ_(1B) is changed from “E” to “L”. Therefore, the potential at the gate terminal N₂₀₁ of the transistor P₂₀₃ is changed from “L” to “E”, and also the potential at the gate terminal N₂₀₂ of the transistor P₂₀₄ is changed from “E” to “L”. Then, at the time point when the potential between the gate terminal N₂₀₁ and the drain terminal N₂₀₂ or the source terminal N₂₀₃ of the transistor P₂₀₃ has become lower than the threshold voltage of the transistor P₂₀₃, the transistor P₂₀₃ is switched from the on state to the off state. In addition, at the time point when the potential between the gate terminal N₂₀₂ and the drain terminal N₂₀₄ or the source terminal N₂₀₅ of the transistor P₂₀₄ has become larger than the threshold voltage of the transistor P₂₀₄, the transistor P₂₀₄ is switched from the off state to the on state, and also the conduction is established between the drain terminal N₂₀₄ and the source terminal N₂₀₅ of the transistor P₂₀₄.

Next, when the operation proceeds from a period of time of (II) to a period of time of (III), the level of each of the clock signals φ_(1A) and φ_(2B) remains “E”, and the level of the clock signal φ_(1B) remains “L”, and also the level of the clock signal φ_(2A) is changed from “H” to “L”. Therefore, the potential at the gate terminal N₂₀₃ of the transistor P₂₀₁ is changed from “H” to “L”, and hence at the time point when the potential between the gate terminal N₂₀₃ and the drain terminal N₂₀₁ or the source terminal N₂₀₂ of the transistor P₂₀₁ has become larger than the threshold voltage of the transistor P₂₀₁, the transistor P₂₀₁ is switched from the off state to the on state, a current is caused to flow from the drain terminal N₂₀₁ of the transistor P₂₀₁ to the drain terminal N₂₀₂ of the transistor P₂₀₂, and the potential at the drain terminal N₂₀₂ of the transistor P₂₀₂ is raised.

Next, when the operation proceeds from a period of time of (III) to a period of time of (IV), the level of each of the clock signals φ_(1A) and φ_(2B) remains “H”, and the level of the clock signal φ_(1B) remains “L”, and also the level of the clock signal φ_(2A) is changed from “L” to “H”. Therefore, the potential at the gate terminal N₂₀₃ of the transistor P₂₀₁ is changed from “L” to “H”, and also the transistor P₂₀₁ is switched from the on state to the off state.

Next, when the operation proceeds from a period of time of (IV) to a period of time of (V), the level of each of the clock signals φ_(2A) and φ_(2B) remains “E”, and also the level of the clock signal φ_(1A) is changed from “H” to “L”, and the level of the clock signal φ_(1B) is changed from “L” to “H”. Therefore, the potential at the gate terminal N₂₀₁ of the transistor P₂₀₃ is changed from “H” to “L”, and the potential at the gate terminal N₂₀₂ of the transistor P₂₀₄ is changed from “L” to “H”, and at the time point when the potential between the gate terminal N₂₀₁ and the drain terminal N₂₀₂ or the source terminal N₂₀₃ of the transistor P₂₀₃ has become larger than the threshold voltage of the transistor P₂₀₃, the transistor P₂₀₃ is switched from the off state to the on state, and the conduction is established between the drain terminal N₂₀₂ and the source terminal N₂₀₃ of the transistor P₂₀₃. In addition, at the time point when the potential between the gate terminal N₂₀₂ and the drain terminal N₂₀₄ or the source terminal N₂₀₅ of the transistor P₂₀₄ has become smaller than the threshold voltage of the transistor P₂₀₄, the transistor P₂₀₄ is switched from the on state to the off state.

Next, when the operation proceeds from a period of time of (V) to a period of time of (VI), the level of each of the clock signals φ_(2A) and φ_(1B) remains “H”, the level of the clock signal φ_(1A) remains “L”, and the level of the clock signal φ_(2A) is changed from “E” to “L”. Therefore, the potential at the gate terminal N₂₀₅ of the transistor P₂₀₂ is changed from “H” to “L”, and hence at the time point when the potential between the gate terminal N₂₀₅ and the drain terminal N₂₀₂ or the source terminal N₂₀₄ of the transistor P₂₀₂ has become larger than the threshold voltage of the transistor P₂₀₂, the transistor P₂₀₂ is changed from the off state to the on state, a current is caused to flow from the drain terminal N₂₀₂ to the source terminal N₂₀₄ of the transistor P₂₀₂, and the potential at the source terminal N₂₀₄ of the transistor P₂₀₂ is raised.

Next, when the operation proceeds from a period of time of (VI) to a period of time of (VII), the level of each of the clock signals φ_(2A) and φ_(1B) remains “H”, and also the level of the clock signal φ_(1A) remains “L”, and the level of the clock signal φ_(2B) is changed from “L” to “H”. Therefore, the potential at the gate terminal N₂₀₅ of the transistor P₂₀₂ is changed from “L” to “H”, and hence at the time point when the potential relation between the gate terminal N₂₀₅ and the drain terminal N₂₀₂ or the source terminal N₂₀₄ of the transistor P₂₀₂ has become smaller than the threshold voltage of the transistor P₂₀₂, the transistor P₂₀₂ is switched from the on state to the off state.

In the above-mentioned operation, with respect to the transistors P₂₀₁ and P₂₀₃ and the capacitors C₂₀₁ and C₂₀₂ for example, when the potential at the node N₂₀₁ is “E” and the potential at each of the nodes N₂₀₂ and ₂₀₃ is “L” (for a period of time of (III)), the transistor P₂₀₁ is turned on, a current is caused to flow from the node N₂₀₁ to the node N₂₀₂, and the potential at the node N₂₀₂ is further raised as compared with its potential at the time before the transistor P₂₀₁ is turned on. Subsequently, when the level of the clock signal φ_(1A) goes to “L”, the level of the clock signal φ_(1B) goes to “H”, and also the potential at the node N₂₀₁ goes to “L” and the potential at the node N₂₀₂ goes to “E” (for a period of time of (i)), the transistor P₂₀₃ is turned on, and the node N₂₀₂ becomes conductive with the node N₂₀₃. Therefore, the potential difference between the source and the gate of the transistor P₂₀₁ becomes zero. At this time, although the potential at the node N₂₀₁ becomes lower than that at the node N₂₀₂, no current is caused to flow between the nodes N₂₀₂ and N₂₀₁ since the transistor P₂₀₁ is turned off. In addition, the potential at the node N₂₀₂ becomes higher than the potential existing when the transistor P₂₀₁ is in the on state by about V_(φ)·C/(C+Cs) as shown in the expression (1), and therefore the potential at the node N₂₀₂ becomes higher than the potential existing when the “H” state has been obtained.

The above-mentioned operation is also applicable to the circuit blocks PCH₀₂ to PCH₀₄, and hence the output potential of the circuit block located at preceding stages or closer to the output terminal becomes higher in the positive direction. That is, the semiconductor booster circuit according to the seventh embodiment is the positive high voltage generating circuit employing the P-channel MOS transistors.

Incidentally, in the semiconductor booster circuit according to the seventh embodiment, for example, since the potential at the substrate terminal SUB₁ is higher than the potential at the sources or drains of the transistors P₂₀₁ and P₂₀₂, the absolute value of the threshold voltage is increased due to the substrate effect, and hence both the transistors P₂₀₁ and P₂₀₂ are difficult to be turned on, or there is a possibility that the on-current becomes small. However, the whole substrate potentials are divided into the two potentials, i.e., the potential at SUB₁ and the potential at SUB₂, whereby the increase of the threshold voltage due to the substrate bias effect is reduced. If the substrate potentials are divided into four blocks and the potential of each block is controlled, the integration becomes poor but the increase of the threshold voltage due to the substrate bias effect can be further reduced.

In the semiconductor booster circuit according to the seventh embodiment, since the voltage drop when transferring the electric charges from the preceding stage to the subsequent stage can be made substantially zero, the larger boosting capability is obtained as compared with the sixth embodiment. In particular, in the case where the power supply voltage Vdd is about 0.8V to 2.0V, the difference in boosting capability between the sixth embodiment and the seventh embodiment becomes remarkably large. For example, in the case where the power supply voltage Vdd is about 0.8V to 2.0V, the number n of stages of the booster circuit required for obtaining a desired output voltage needs to be greatly increased in the booster circuit of the sixth embodiment due to the voltage drop when transferring the electric charges from the preceding stage to the subsequent stage, but in the booster circuit of the seventh embodiment, it is not required at all. For example, in the case where the power supply voltage Vdd is 2.0V, in the sixth embodiment, the number of stages of the booster circuit required for obtaining the output voltage V_(POUT) of 23Vis 20 whereas in the booster 15 circuit of the seventh embodiment, the required number of stages is only 12.

On the other hand, the booster circuit of the sixth embodiment is advantageous as compared with the booster circuit of the seventh embodiment in that the configuration is simpler and also only two kinds of clock signals are required.

Incidentally, it is to be understood that in the above-mentioned embodiments, the various changes may be made. For example, the number of stages of the booster circuit is not limited to four in the above-mentioned embodiment, and hence it may be set to any value determined in accordance with the voltage to be boosted, the circuit scale and the like. In addition, the N-channel depletion type MOS transistors M₁₀₁ to M₁₀₈ are exemplarily employed as the transistors constituting the booster circuit in the sixth embodiment and also the P-channel MOS transistors P₂₀₁ to P₂₀₄ are exemplarily employed as the transistors constituting the booster circuit in the seventh embodiment. However, as for those transistors, other transistors such as N-channel enhancement type MOS transistors may also be employed. For example, the N-channel MOS transistors M₁₀₁ to M₁₀₈ in the sixth embodiment may be substituted by P-channel MOS transistors which are formed in the N type well region, and also the power supply terminal N₁₂₀ may be grounded to provide the negative high voltage generating circuit. In addition, the P-channel MOS transistors P₂₀₁ to P₂₀₄ in the seventh embodiment may be substituted by N-channel MOS transisters which are formed in the P type well region to provide the negative high voltage generating circuit.

In the sixth and seventh embodiments, the substrate terminals of the MOS transistors constituting the booster circuit are divided into the necessary groups, and also are controlled to the different potentials for the groups, whereby it is possible to prevent the substrate bias effect from occurring. Therefore, the high boosting capability can be obtained and also the increase of the circuit area can be kept to a minimum. 

1. A semiconductor booster circuit, comprising: a plurality of stages each having a MOS transistor, a first capacitor and a second capacitor, said MOS transistor being formed in a well and having a drain, a source and a gate, said first capacitor having one terminal connected to the drain of said MOS transistor, and said second capacitor having one terminal connected to the gate of said MOS transistor; said plurality of stages being connected in series between an input terminal and an output terminal by connecting said MOS transistors of said plurality of stages in cascade; means for applying a power supply voltage to the drain of said MOS transistor in the stage closest to said input terminal so that said power supply voltage is boosted by said MOS transistor in each of said stages thereby generating a boosted voltage which is higher than said power supply voltage at the source of said MOS transistor in the stage closest to said output terminal; a first clock signal generating means for generating a first clock signal, and supplying said first clock signal to another terminal of said first capacitor in each stage; and a second clock signal generating means for generating a second clock signal which has a larger amplitude than said power supply voltage, and supplying said second clock signal to another terminal of said second capacitor in each of said plurality of stages; wherein in each of said plurality of stages, the source of said MOS transistor is electrically connected to the well in which said MOS transistor is formed; and the wells of said plurality of stages are electrically insulated from each other.
 2. A semiconductor booster circuit, comprising: a plurality of stages, each having a MOS transistor and a capacitor, said MOS transistor being formed in a well and having a drain, a source and a gate, and said capacitor having one terminal connected to the drain of said MOS transistor, wherein said plurality of stages are connected in series between an input terminal and an output terminal by connecting said MOS transistors of said plurality of stages in cascade; a power supply voltage is applied to the drain of said MOS transistor in the stage closest to said input terminal so that said power supply voltage is boosted by said MOS transistor in each of said stages thereby generating a boosted voltage which is higher than said power supply voltage at the source of said MOS transistor in the stage closest to said output terminal; wherein in each of said plurality of stages, the source of said MOS transistor is electrically connected to the well in which said MOS transistor is formed; and the wells of said plurality of stages are electrically insulated from each other.
 3. A semiconductor booster circuit according to claim 2, wherein the wells of said plurality of stages are N-type wells formed in a semiconductor substrate, and are electrically insulated from each other; and said MOS transistors of said plurality of stages are P-channel MOS transistors formed in said N-type wells, respectively.
 4. A semiconductor booster circuit according to claim 2, wherein each of said plurality of stages further has: another capacitor having one terminal connected to the gate of said MOS transistor; and another MOS transistor connected between the gate and source of said MOS transistor, and having another gate connected to the one terminal of said capacitor.
 5. A semiconductor booster circuit according to claim 4, wherein in two adjacent stages of said plurality of stages, a pair of first clock signals, having opposite phase with each other, are inputted to another terminal of said capacitors, respectively; and a pair of second clock signals are inputted to another terminal of said another capacitors in different timing with each other, respectively.
 6. A semiconductor booster circuit according to claim 2, wherein in one stage of said plurality of stages and a subsequent stage of said one stage, the gate of said MOS transistor of said one stage is connected to the one terminal of said capacitor of said subsequent stage; and in two adjacent stages of said plurality of stages, a pair of clock signals, having opposite phase with each other, are inputted to another terminals of said capacitors, respectively.
 7. A semiconductor booster circuit having MOS transistors connected in cascade, comprising: a plurality of stages each having a MOS transistor, a first capacitor one terminal of which is connected to a drain of said MOS transistor, and a second capacitor one of terminal of which is connected to a gate of said MOS transistor, said plurality of stages being connected in series between an input terminal and an output terminal by connecting said MOS transistors of said plurality of stages in cascade; means for applying a power supply voltage to the drain of said MOS transistor in the stage closest to said input terminal so that said power supply voltage is boosted by said MOS transistor in each of said stages thereby generating a boosted voltage which is higher than said power supply voltage at the source of said MOS transistor in the stage closest to said output terminal; a first clock signal generating means for generating a first clock signal, and supplying said first clock signal to another terminal of said first capacitor; and a second clock signal generating means for generating a second clock signal which has a larger amplitude than said power supply voltage, and supplying said second clock signal to another terminal of said second capacitor; wherein in each of said plurality of stages, a source of said MOS transistor is electrically connected to a well in which said MOS transistor is formed; and the wells of said plurality of stages are electrically insulated from each other.
 8. A semiconductor booster circuit according to claim 7, wherein the wells of said plurality of stages are N-type wells formed in a semiconductor substrate, and are electrically insulated from each other; and said MOS transistors of said plurality of stages are P-channel MOS transistors formed in said N-type wells, respectively.
 9. A semiconductor booster circuit according to claim 7, wherein each of said plurality of stages further has another MOS transistor which is connected between the gate and source of said MOS transistor and has a gate connected to the one terminal of said first capacitor.
 10. A semiconductor booster circuit according to claim 7, wherein in two adjacent stages of said plurality of stages, said first clock signals inputted to the another terminals of said capacitors, respectively, have opposite phase with each other.
 11. A semiconductor booster circuit having MOS transistors connected in cascade, comprising: a plurality of stages each having a first MOS transistor, a second MOS transistor, a first capacitor having one terminal which is connected to a drain of said first MOS transistor, and a second capacitor having one terminal which is connected to a source of said second MOS transistor; wherein in each of said stages, said first and second MOS transistors are connected in series to constitute a series circuit; said series circuits, which are constituted in said plurality of stages, respectively, are connected in series between input and output sides of said plurality of stages; a power supply voltage is applied to the drain of said first MOS transistor in the stage closest to said input side so that said power supply voltage is boosted by said first and second MOS transistors in each of said stages thereby generating a boosted voltage which is higher than said power supply voltage at the source of said second MOS transistor in the stage closest to said output side; said plurality of stages are divided into at least two groups; said first and second MOS transistors included in the stages of each group are formed in a well which is formed in a semiconductor substrate; and electrical potentials respectively applied to said wells in said groups are controlled independently of one another.
 12. A semiconductor booster circuit according to claim 11, wherein when said booster circuit is a circuit for generating a positive high voltage, a potential which is applied to said well in one group closer to the output side of said plurality of stages is higher than another potential which is applied to said well in the other group.
 13. A semiconductor booster circuit according to claim 12, wherein said well of each group is an N-type well which is formed in said semiconductor substrate; said first and second MOS transistors are P-channel transistors formed in said N-type well; and said N-type wells which are formed in said groups, respectively, are electrically insulated from each other.
 14. A semiconductor booster circuit according to claim 11, wherein in each group, said well is connected to the source of one of said first and second MOS transistors which is nearest to the input side. 